The present invention is directed to a method and apparatus for optimizing a rotating velocity of a magnetic medium, and in particular to a method and apparatus for controlling the rotating velocity in response to environmental conditions.
One of the key components of some electrical devices is a place to store and read data. For example, compact disk players read data, such as music, from a plastic disk. Another example is a VCR which reads data from a tape. Computer systems also store and read large amounts of data. Typically computer systems employ a number of storage means to store data. One of the places where a computer can store data is in a disk drive which is also called a direct access storage device.
Although this invention is not limited to a direct access storage device one will be described by way of example. A disk drive or direct access storage device includes several disks which look similar to records used on a record player or compact disks which are used in a CD player. The disks are stacked on a spindle, much like several records awaiting to be played. In a disk drive, however, the disks are mounted to the spindle and spaced apart so that the separate disks do not touch each other.
The surface of each disk is uniform in appearance. However, in actuality, each of the surfaces is divided into portions where data is stored. There are a number of tracks situated in concentric circles like rings on a tree. Each track in a disk drive is further subdivided into a number of sectors which is essentially just one section of the circumferential track.
Storage of data on a magnetic disk entails magnetizing portions of the disk in a pattern which represents the data. To store data on a disk the disk is magnetized. In order to magnetize the magnetic layer, a small ceramic block which contains a magnetic transducer known as a write element is passed over the surface of the disk. More specifically, the write element is flown at a height of approximately six millionths of an inch from the surface of the disk and is flown over the track as the write element is energized to various states causing the track below to be magnetized to represent the data to be stored. In some applications, the write element is the same as the read element. Other applications use a separate write element and a separate read element.
To retrieve data stored on a magnetic disk, a read element located in close proximity to the write element is flown over the disk. The magnetized portions of the disk provide a signal from the read element. By looking at output from the read element, the data can be reconstructed and then used by the computer system.
Like a record, both sides of a disk are generally used to store data or other information necessary for the operation of the disk drive. Since the disks are held in a stack and are spaced apart from one another, both the top and the bottom surface of each disk in the stack of disks has its own read element and write element. This would be comparable to having a stereo that could play both sides of a record at once. Each side would have a stylus which played the particular side of the record.
Disk drives also have something that compares to the tone arm of a stereo record player. There are two types of disk drives, rotary and linear. Rotary disk drives have a tone arm that rotates much like a record player. The tone arm of a rotary disk drive, termed an actuator arm, holds all the transducers or read/write elements, one head for each surface of each disk supported in a structure that looks like a comb. Sometimes the structure is called an E-block. Like a tone arm, the actuator arms rotate so that the read element and write element attached to the actuator arm can be moved to locations over various tracks on the disk. In this way, the write element can be used to magnetize the surface of the disk in a pattern representing the data at one of several track locations. The read element is used to detect the magnetized pattern on one of the tracks of a disk. For example, the needed data may be stored on two different tracks on one particular disk, so to read the magnetic representations of data, the actuator arm is rotated from one track to another track.
It should be noted that this invention is not limited to use in disk drives using magnetic media but is useful in any device having rotating media. In this particular application, where magnetic media is described as an example it should be recognized that the invention would be useful in other storage devices which have different types of media or read and write elements.
The use of direct access storage devices (DASD) in portable systems has increased significantly over the past several years. For example, magnetic hard disk drives are commonly incorporated into portable computers. The portable nature of the computer subjects the storage device to a variety of different operating environments and conditions. For example, the portable computer may be used in an airplane, thereby subjecting the storage device to substantial variations in atmospheric pressure, turbulent vibrations and the like.
The various operating environments and conditions in which a storage device is used can have significant adverse effects on the operation of the storage device. For example, a conventional magnetic disk drive which employs a rotating disk may be used in a portable computer. The disk drive uses a transducer, formed as part of a magnetic head assembly, to read information from and write information to the disk. The magnetic head assembly typically includes a slider having aerodynamic properties sufficient to allow the head assembly to fly in close proximity over the surface of the disk forming an air bearing between the transducer and the disk surface. A flexure arm couples the transducer/slider arrangement to an actuator arm. The flexure arm is spring loaded and opposes the aerodynamic lift of the slider to control a flying height over the surface of the disk. In a typical disk drive, the air bearing forms a head to disk spacing on the order or 50 nanometers (nm). When a conventional disk drive is incorporated into a portable computer used on an airplane, the lower cabin pressure reduces the lift capacity of the slider thereby reducing the air bearing between the head and disk. As the flying height is reduced, the likelihood of a head to disk crash increases.
A head to disk crash is a catastrophical event for a disk drive. It renders the disk drive useless, since access to data is no longer possible. This data loss can have severe economic implications for the user. It is therefore, of critical importance to minimize the likelihood of a head to disk crash.
Other environmental conditions can also adversely effect the operation of a storage device using a rotating storage medium. Generally hard disk drives used for portable applications are physically small and rugged devices employing embedded servo control for positioning of the actuator. A portable disk drive employing embedded servo can be significantly impaired by external shocks and vibrations. Embedded servo pattern information is typically provided on the surface of the disk in a number of servo sectors extending generally outward from the center of the disk at regularly spaced intervals around the disk. Because the portion of the disk used for the embedded servo information cannot be used to store data, it is desirable to reduce the number of servo sectors (i.e., space the servo information on the disk as far apart as possible). At each servo sector the transducer, which tends to drift off of a desired track location between servo sectors, may be realigned over the center of the track. The farther apart the servo information is spaced, however, the more likely it is that the head will move sufficiently off of the proper track location to cause an error. External shocks and vibrations tend to cause more movement in the transducer between servo sectors increasing the likelihood that the transducer will move off of the track prior to reading the next servo sector.
As the above examples illustrate, current storage devices fail to adequately provide a mechanism to account for the various external influences resulting from the environment in which the disk drive is operating. Accordingly there exists a need for an improved storage device which accounts for the environment in which the device is operating.